As discussed in the background section of U.S. Pat. No. 5,521,392 (Kennedy et al.), commonly owned with the present invention, there are numerous substances which are sensitive to light energy. The substances of interest generally fall into two classes. The first class comprises substances which undergo polymerization in response to applied light energy. The second class comprises substances which produce a “singlet oxidation molecule” in response to applied light energy. The second class of substances can be found in “photodynamic therapy” or “phototherapy” applications, while the first class of photo-sensitive substances are typically found in UV polymerization and photochemical curing of adhesives.
It is known that the time it takes to cure a photosensitive material, such as an adhesive, ink or other coating is influenced by two principal factors. The first factor encompasses the type of photosensitive material and amount which is required for the application. Once determined for the particular application, this factor remains fixed for the application. The second factor affecting the cure time involves the amount of light energy being delivered to the cure the adhesive. It is also known that the intensity of the light produced by the light source may decrease over the life of the source or be affected by environmental conditions. If the light source degrades, so will the amount of light energy being delivered to cure the adhesive and therefore a longer exposure time is needed to properly cure the adhesive.
In some applications, it may be desirable to adjust the intensity level instead of the exposure time in order to provide a light energy output which is optimum for a particular curing application. Moreover, it is usually desirable to be able to compensate for changes in the light intensity levels caused by the aforementioned environmental changes or the light source degrading throughout its useful life.
Similarly, for printing applications, curing of a uv-curable ink is also influenced by two principle factors. The first factor encompasses the formulation of the ink and amount of UV light which is required for the application, i.e. full cure, pinning, coating, or other application. Once determined for the particular application, this factor remains fixed for the application. The second factor affecting the cure time involves the amount of UV light energy being delivered to cure the UV ink. It is also known that the intensity of the UV light produced by a UV light source, i.e., a conventional lamp, will decrease over the life of the light source. While a UV LED light source does not degrade in the same way, the intensity of both types of UV light source is sensitive to changes in environment, such as temperature changes. If the intensity decreases, the amount of UV light energy being delivered decreases and the intensity or exposure may need to be adjusted.
In some UV curing applications (i.e., full cure, pinning, coating, etc), it may be desirable to adjust the UV intensity level instead of the exposure time in order to provide a UV light energy output which is optimum for the particular UV curing application. Moreover, it is usually desirable to be able to compensate for changes in the UV light intensity levels produced by the UV LED light source which degrades throughout its useful life. U.S. Pat. No. 5,521,392 describes a light curing system for use with such photosensitive materials which provides continuous intensity monitoring and adjusts the exposure time or intensity level, or both, to compensate for output degradation in the light source and thereby provide a constant light energy output, from the curing system, for a given iris setting. The intensity level is measured at the output of a light delivery means of the light curing system. This signal is used by a controller to calibrate the internal continuous monitoring sensor. This allows the internal sensor to be calibrated periodically against a NIST traceable device.
It is desirable to calibrate each radiometer off-site according to industry standards, so each radiometer is detachable so that it can be removed for calibration and replaced temporarily with a spare radiometer which has already been calibrated. In a production setting, there may be many light curing systems and hence many radiometers, especially when the substitutes are included, Calibration of these multiple radiometers is not only time-consuming but also may lead to undesirable differences between the output intensity levels.
In a production setting, there is usually a need for a high degree of consistency between the output intensity levels of the curing systems. Different rates of degradation of the light sources, and transmission losses of the light guides, the different responses of light sources to environmental conditions, as well as different rates of drift of the radiometers, may lead to undesirable differences between output intensity levels of the different light curing systems.
The problem is compounded where several light curing systems are preset to the same output intensity or irradiance setting. As the light curing system degrade or respond differentially, these settings may no longer be the same and may require re-calibration, typically at least once per week and possibly every day.